The present invention generally relates to a rear projection screen for use in displaying an image projected from an image source. More particularly, the invention is directed to a rear projection screen which utilizes a front surface lenticular array to distribute light from the image source. In this regard, an important aspect of the present invention concerns a novel lenticular lens construction having a side reflective region and a tip refracting region and a novel shape for the tip refracting region which distributes light from the projection light source horizontally through virtually 180.degree. with perceptually uniform luminance.
Rear projection screens are utilized in various types of equipment, such as projection-based radar displays, flight simulators, avionic displays, traffic control lights, microfilm readers, video games, projection-based video monitors, and rear-projection film media displays for generating a user-viewable display. In such applications, an image source located behind the screen projects light forward along a projection axis toward the screen to form an image at the plane of the screen which is distributed to viewers on the front side of the screen.
Where multiple viewers are present, such viewers are typically spread out horizontally, and distribution of light through a large horizontal angle is desirable. This is particularly true in rear screen projection television receivers, where multiple viewers are generally present and seated throughout a relatively wide horizontal angle relative to the screen.
One problem encountered in rear projection systems is that more light energy is projected along the projector axis than elsewhere, so that the closer an observer is to the projection axis, the brighter the image appears to him. Rear screen projection color video units typically utilize three cathode ray tubes, one for each of the primary red, green, and blue colors, which project through individual projection lens assemblies onto the projection screen. In a typical horizontal arrangement of the cathode ray tubes, the green cathode ray tube is typically positioned in the center along the projection axis, and the red and blue cathode ray tubes are positioned with their optical axes offset with respect to the green optical axis at an angle of 5.degree. to 11.degree.. These offsets, unless corrected for by the screen, give rise to a phenomenon known as color shift. That is, if the red, green and blue light luminances for the center of the viewing audience are normalized, the ratio of the luminances will vary with angular position throughout the horizontal viewing plane. Thus, the viewer's perception of the image depends upon the viewer's horizontal placement with respect to the screen.
In addition, when rear projection screens are operated in and exposed to ambient light, the contrast of the projected image is affected by the reflection of such light at the front side of the screen. Thus, it is desirous to reduce reflection of such light at the front side of the screen. Various masking techniques, wherein a black non-reflective surface is interposed between the lenticules, have been proposed for minimizing light reflection. One such technique, which is particularly effective is described in the copending application of Douglas A. Stanton, entitled "Improved Blackened Optical Transmission System", Ser. No. 567,022, filed concurrently herewith.
Various rear projection screens have been proposed to increase the horizontal viewing angle range. One such system, which is described in U.S. Pat. No. 3,578,841 to William R. Elmer, utilizes a screen having a rear surface, such as a Fresnel lens, capable of collimating light from the image source into parallel rays, and a front surface formed with a vertically-ribbed spreading lens to spread light throughout a specified horizontal viewing angle. The shape of the vertical ribs of the spreading lens is defined by an equation dependent upon the desired angle of spread from the center line. However, ribs drawn in accordance with such an equation become seriously inefficient when spreading light through more than a 60.degree. viewing angle, i.e. 30.degree. on either side of the center line, and necessitate positioning such ribs between a pair of sharply pointed side ribs to increase the viewing angle to 90.degree., i.e. 45.degree. on either side of the center line.
Other types of rear projection screens are described in an article by Yoshito Miyatake and Yoshitomi Nagaoka, entitled "One-Piece Type, Super Wide Angle Rear Projection Screen," published in Japan Display '83-587. One screen, identified as Phase-III, utilizes a trapezoidal lenticule having an upper surface the same in cross-section as that of a conventional cylindrical lenticule, and side surfaces which are flat and slightly tilted to the center axis of the rear screen, and alternating conventional cylindrical lenticules. According to the article, Phase-III screens have a horizontal viewing angle of .+-.60.degree. but are prone to rapid color-shifts when utilized as rear projection televison screens. The second screen, identified as Phase-IV, utilizes a trapezoidal lenticule, the surface of which is formed into two cylindrical surfaces and alternating standard cylindrical lenticules. The standard cylindrical lenticules are interposed between the two cylindrical surfaces of the trapezoidal lenticules. According to the article, such a configuration decreases the rapid color-shift of Phase-III while allowing relatively large horizontal viewing angles. However, the resulting Phase-IV lenticule is extremely foreshortened in cross-section, having a greater width than height, with the width of the tip portion only slightly less than the overall width of the lenticule, which limits the extent of black masking possible and which also limits the amount by which the light can be spread uniformly in the horizontal direction. The present invention is directed to a display screen construction which avoids these shortcomings.